1. Field of the Invention
This invention relates generally to electrode assemblies useful in electrochemical cells and, more specifically, this invention relates to electrode assemblies for consumable metal electrode/aqueous alkaline electrolyte cells useful in the generation of electrical energy.
2. Description of the Related Art
Electrochemical cells utilizing monopolar or bipolar cell designs having reactive metal electrodes supported on a substrate current collector are well known. See, for example, Momyer et al, U.S. Pat. No. 4,269,907 (May 26, 1981), the disclosure of which is hereby incorporated by reference, wherein cells including an aqueous electrolyte, an anode of an alkali metal, such as lithium, for example, a cathode spaced from the anode, and an intercell electrical connector are disclosed. In such bipolar cells, the cathode may comprise an electrochemically active material, such as silver oxide, and the electrolyte is an alkaline aqueous solution.
Momyer et al also disclose an electrochemical cell stack battery comprising a plurality of bipolar cells connected in series.
The operation of such cells involves the following reactions, which, for illustrative purposes, utilize lithium as the reactive anode and aqueous lithium hydroxide as the electrolyte.
A. Anode Reaction
Electrochemical Dissolution EQU Li.fwdarw.Li.sup.+.sub.(aq) +e.sup.- ( 1)
Formation of Insulating Film on Anode EQU Li.sup.+.sub.(aq) +OH.sup.-.sub.(aq) .fwdarw.LiOH.sub.(aq) ( 2) EQU LiOH.sub.(aq) .fwdarw.LiOH.sub.(s) ( 3)
Direct Corrosion/Parasitic Reaction EQU Li+H.sub.2 O.fwdarw.LiOH.sub.(aq) +1/2H.sub.2 (g) ( 4)
B. Cathode Reaction
Reduction of Water EQU H.sub.2 O+e.sup.- .fwdarw.OH.sup.- +1/2H.sub.2 (g) ( 5)
(aq) represents an ion dissolved in water and (s) represents a solid salt.
Reactions (1) and (5) are necessary for the generation of electricity. Reactions (2) and (3) serve to produce a porous insulating film which forms on the anode and protects it. Reaction (4) is a parasitic reaction which generates no useful current.
At the start of discharge, the lithium anode surface is smooth and the gap between the anode and cathode is uniform in size. During discharge, the above-identified reactions serve to remove lithium from the active surface in a non-uniform manner and, as the discharge progresses, the surface of the lithium becomes highly irregular due to local variations in the electrolyte flow velocity, temperature, concentration and/or current density. Toward the end of discharge, when the average lithium thickness is small, some thin spots in the lithium layer will break through the lithium electrode and expose the substrate current collector.
The current collector is made of an electrically conductive metallic substrate, such as nickel. When such a nickel substrate is exposed to the electrolyte, it effectively acts as a cathode for the generation of hydrogen according to equation (4), above. This discharge, however, produces no useful current because the lithium anode and the nickel substrate, now acting as a cathode, are electrically shorted together.
Thus, when the nickel substrate is exposed, a "shorted cell" is created and the current density on the remaining lithium is increased as the lithium anode must now support the current delivered by the cell to the external load as well as the short circuit current resulting from the "shorted cell" created on the anode when the substrate is exposed. By increasing the polarization of the lithium, this extra load reduces the overall cell voltage. Additionally, such a "shorted cell" results in the generation of additional hydrogen gas and heat as well as consumption of lithium, which consequently is not available for supplying current to the load.
In torpedo propulsion batteries, the lithium anode is initially quite thin, resulting in exposure of the nickel substrate well before all available lithium has been utilized to produce useful current. Consequently, the discharge time, when at the full design power output, is substantially reduced.